Peel and cut the Floury Potatoes into even chunks, place in a pot of cold water, add a pinch of Salt, bring to a boil and simmer until tender, about 15–20 minutes.
Drain the potatoes, return them to the pot off the heat and mash with a generous knob of Butter and enough Whole Milk to reach a creamy consistency; adjust seasoning with Salt to taste and keep warm.
Thinly slice the Onion and set aside.
Cut the Reindeer Meat into thin strips or bite-sized pieces and season lightly with freshly ground Black Pepper (and a little extra salt if needed).
Heat a large skillet over medium-high heat and add a little butter; when foaming, add the meat in a single layer (work in batches to avoid crowding) and let it brown undisturbed for 1–2 minutes, then stir or flip to brown the other side.
Push the browned meat to one side of the pan, add the sliced Onion to the pan and sauté briefly until softened and translucent, then mix with the meat.
Pour a splash of Water into the skillet to deglaze, scraping up any browned bits, reduce the heat and simmer for a couple of minutes so the meat finishes cooking and the pan juices form a light sauce.
Serve the sautéed reindeer and onions over the creamy mashed potatoes and offer a spoonful of Lingonberry Jam on the side to add a bright, tart contrast.
